The invention pertains to systems for circulation of a liquid solution between a gas exchange apparatus and a subject to which the liquid solution is being supplied.
One such system is used for extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass, in which venous blood is circulated through an extracorporeal oxygenation system and returned to the subject's circulatory system. Venous blood received through a venous cannula is transported past a saturation monitor to a bladder reservoir. The reservoir feeds a pump which drives the blood through a membrane oxygenator and heat exchanger, returning it finally to the subject's circulatory system through an output cannula. A bridge circuit may be opened to allow circulation of blood within the oxygenation system to bypass the subject's circulatory system to prevent a pressure buildup in the oxygenation branch of the ECMO circuit or to create a flow through the oxygenator which is greater than the flow to the patient.
Two forms of ECMO are currently in use. In venoarterial ECMO, the output conduit of the oxygenation system empties into an artery through an arterial cannula, placed typically in the carotid artery. In venovenous ECMO, the output conduit empties into the patient's venous circulation, typically through a cannula in the femoral vein, jugular vein, or right atrium.
An inherent limitation of all ECMO systems is that the rate of oxygenation depends on the flow of venous blood into the oxygenation system. This flow results primarily from circulatory pressure, and is limited by the impedance of the venous cannula. ECMO systems are further limited by their pumping means. The roller pumps and centrifugal pumps which are presently employed are adequate to provide continuous circulation, but cannot provide pulsatile flow. Roller pumps have the additional disadvantage of not allowing "slip" in the event of pressure buildup in the oxygenation branch of the ECMO circuit.
A disadvantage particular to renovenous ECMO is that a portion of the oxygenated blood provided to the subject is taken up again by the venous intake cannula before the blood can be circulated through the subject's circulatory system. This decreases systemic oxygen delivery and reduces the driving pressure (oxygen pressure drop) across the oxygenation membrane, thereby decreasing the efficiency of ECMO. Another disadvantage of venovenous ECMO is that cannulation of the femoral vein with a sufficiently large cannula has proven difficult in infants. While some success has been achieved using a double lumen cannula placed into the jugular vein, the use of the double lumen cannula may increase the wasteful recirculation of oxygenated blood through the oxygenation circuit and greatly reduces the potential diameter of each lumen.
A second apparatus for circulation of a liquid solution between a gas exchange apparatus and a subject is used in tidal liquid ventilation (TLV). TLV uses a liquid solvent such as a perfluorocarbon as a medium for the exchange of oxygen and carbon dioxide within a patient's lungs. A volume of oxygenated solvent is propelled through an endotracheal tube into the subject's lungs. The liquid expands the lungs, which subsequently recoil and expel the oxygen-depleted solvent back through the endotracheal tube and into the TLV circuit. The solvent is then reoxygenated and recirculated.
A disadvantage of present TLV systems is that they lack fine control of the flow rate during inspiration and expiration. A second disadvantage is that they cannot accommodate the long term need for slip. Present systems are only capable of providing a predetermined volume of fluid, and cannot compensate when this volume exceeds the patient's lung capacity.